Mercaptosilane-Passivated CuInS2 Quantum Dots for Luminescence

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Mercaptosilane-Passivated CuInS2 Quantum Dots for Luminescence Thermometry and Luminescent Labels Riccardo Marin, Alvise Vivian, Artiom Skripka, Andrea Migliori, Vittorio Morandi, Francesco ENRICHI, Fiorenzo Vetrone, Paola Ceroni, Carmela Aprile, and Patrizia Canton ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00317 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 2, 2019

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ACS Applied Nano Materials

Mercaptosilane-Passivated CuInS2 Quantum Dots for Luminescence Thermometry and Luminescent Labels Riccardo Marin,a,b,† Alvise Vivian,c Artiom Skripka,b Andrea Migliori,d Vittorio Morandi,d Francesco Enrichi,a,e Fiorenzo Vetrone,b,f Paola Ceroni,g Carmela Aprilec and Patrizia Cantona*

a. Department of Molecular Sciences and Nanosystems, Università Ca' Foscari, Venezia, Via Torino 155/B - 30172 Venezia-Mestre, Italy. b. Institut National de la Recherche Scientifique, Centre Énergie, Matériaux,

Télécommunications

(INRS

–

EMT),

Université

du

Québec, 1650 Boul. Lionel-Boulet, Varennes, Québec, J3X 1S2, Canada. c. Unit

of

Nanomaterials

Chemistry,

University

of

Namur,

Department of Chemistry, Rue de Bruxelles 61 – 5000 Namur, Belgium.

ACS Paragon Plus Environment

1

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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d. CNR-IMM Sezione di Bologna Via P. Gobetti, 101 - 40129 Bologna, Italy. e. Division

of

Materials

Science,

Department

of

Engineering

Sciences and Mathematics, Luleå University of Technology, 971 87 Luleå, Sweden. f. Centre

for

Self-Assembled

Chemical

Structures,

McGill

University, Montréal, Québec, H3A 2K6, Canada. g. Department of Chemistry, “Giacomo Ciamician”, University of Bologna, Via Selmi 2, 40126 Bologna, Italy. †

Present

address:

Department

of

Chemistry

&

Biomolecular

Sciences, University of Ottawa, 10 Marie Curie St., Ottawa (ON) K1N 6N5 (Canada)

KEYWORDS. CuInS2, silane, quantum dots, composites, luminescent films, thiolates, luminescence thermometry

ABSTRACT. Bright and non-toxic quantum dots (QDs) are highly desirable in a variety of applications, from solid-state devices to luminescent probes in assays. However, the processability of these species is often curbed by their surface chemistry, which limits their dispersibility in selected solvents. This renders a surface

modification

step

often

mandatory

to

make

the

QDs


2

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compatible with the solvent of interest. Here, we present a new synthetic approach to produce CuInS2 QDs compatible with organic polar solvents and readily usable for the preparation of composite materials.

3-mercaptopropyl

trimethoxysilane

(MPTS)

is

used

simultaneously as solvent, sulfur source, and capping agent for the

QD

synthesis.

The

synthesized

QDs

possessed

a

maximum

photoluminescence quantum yield around 6% - reaching approximately 55% after growing a ZnS shell. The partial condensation of MPTS molecules on the surface of QDs was probed by solid-state nuclear magnetic resonance, whose results were used to interpret the interaction of the QDs with different solvents. To prove the versatility of the developed QDs – imparted by the thiolated silane molecules

–

we

prepared

via

straightforward

procedure

two

nanocomposites of practical interest: (i) silica nanoparticles decorated with QDs and (ii) an inexpensive polymeric film with embedded

QDs.

We

further

demonstrate

the

potential

of

this

composite film as a luminescence thermometer operational over a broad

temperature interval, with relative thermal sensitivity

above 1% K-1 in the range of biological interest.

INTRODUCTION Quantum dots (QDs) are amongst the most intensively studied classes of luminescent nanomaterials over the last decades, and the capability of tailoring their optical properties has reached


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a high level of sophistication.1-2 Although this holds true for heavy metal-based binary semiconductors (e.g. CdSe, CdTe, PbS, etc.),1-2

the

development

of

heavy

metal-free

QDs

exhibiting

equally good optical properties and the same degree of optical and chemical tunability remains a challenge to overcome. Although commonly used as a harvesting material in solar cells,3-4 copper

indium

sulfide

(CIS)

has

recently

gained

increasing

attention within the QD community.5-7 The interest in the field of ternary QDs (and CIS in particular) experienced as of late is justified by the possibility of avoiding the use of heavy metalbased materials in QD-based applications (LEDs,8-9 photovoltaics,1011

and biomedicine12-13 to name some). This perspective is at the

heart of why groups are putting significant efforts on the side of materials engineering (i.e. optimization of composition, surface chemistry and synthesis method) as well as the study of the fundamental electronic properties of this class of QDs. With regards to the first aspect, important advancements have been

achieved

in

terms

of

producing

CIS

QDs

with

high

photoluminescence quantum yield (PLQY),14-15 whose emission can be also conveniently tuned over a broad range of wavelengths. This optical flexibility is achieved via alloying,16-17 cation-exchange1819

and growth of passivating shells composed of large band gap

semiconductor materials.20-21 However, most of the synthesis methods developed to produce high quality ternary QDs rely on the use of


4

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hydrophobic capping agents, leading to materials dispersible only in non-polar solvents. Hence, the immediate applicability of the QDs in polar organic environments is curbed. An important goal in the field is to obtain CIS QDs that are readily dispersible in the solvent of choice depending on the specific application sought after. Usually, QDs are transferred between different solvents via a post-synthesis surface modification7, partial

loss

of

the

primary

PLQY.

22

with the drawback of a

Alternatively,

specific

molecules are introduced directly in the reaction environment. These species remain bound to the QD surface at the end of the synthesis, imparting compatibility with the desired solvents.23 The assessment of the suitable molecules to be employed in the synthesis is not a trivial task, since it is also necessary to achieve good control over the reactivity of two different elements, Cu+ and In3+, displaying respectively soft and hard Lewis acid behavior.24 Albeit the challenges, this approach is able to deliver a ready-to-use system that does not require further modification steps. Given the technological relevance of CIS QDs and their relatively

recent

history,

there

is

plenty

of

room

for

the

development of new synthesis methods and improvement of preexisting

ones,

along

with

the

exploration

of

their

possible

applications. In this study, we present a method to synthesize CIS QDs readily dispersible in a variety of solvents (including polar organic media


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Page 6 of 51

such as acetone), which are further shown to be suitable for the preparation thermometry. precursor

of We

nanocomposites accomplished

(3-mercaptopropyl

and this

for

use

using

a

trimethoxysilane

in

luminescence

thiolated -

MPTS)

silica as

the

passivating molecule. A couple of studies have demonstrated the possibility of obtaining MPTS-capped binary and ternary QDs via a post-synthesis ligand exchange process.25-26 Nonetheless, the Scheme 1. Synthesis of the MPTS-passivated CuInS2 QDs (top) and their use in the preparation of two nanocomposites (bottom left – luminescent polymeric film for luminescence thermometry; bottom right – silica nanoparticles decorated with QDs).


6

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production of luminescent CuInS2 QDs using MPTS simultaneously as solvent, capping agent, and sulfur source is yet to be reported. We

obtained

sols

of the

prepared QDs

in solvents

displaying

different polarities, while the growth of a ZnS shell allowed to increase the QDs’ PLQY by almost one order of magnitude, reaching 55%. The possibility of utilizing these QDs in different contexts of practical interest was demonstrated preparing two different nanocomposites (Scheme 1): (i) silica nanoparticles decorated with the synthesized CIS QDs and (ii) a luminescent polymeric film in


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which the QDs were embedded. Specifically, this last nanocomposite film

was

explored

as

luminescence

thermometer

over

a

broad

temperature range. EXPERIMENTAL SECTION Chemicals Copper bromide (CuBr, 98%), indium acetate (In(OAc)3, 99.999%), zinc acetate (Zn(OAc)3, 99.99%), 3-mercaptopropyl trimethoxysilane (MPTS,

95%),

chloroform

(CLF,

>99%),

acetone

(99.5%),

tetraethoxysilane (TEOS, 98%), and polyethylene glycol thiol MW 500 (PEG-SH) were purchased from Sigma-Aldrich. Ammonium hydroxide (NH4OH, 28%) and ethanol (99.8%) were purchased from Fluka. All chemicals

were

of

chemical

grade

and

used

without

further

purification.

QD synthesis Synthesis of MPTS-passivated CIS QDs (core) CIS QDs were prepared mixing the precursor salts (0.2 mmol –28.7 mg– of CuBr and 0.2 mmol –58.4 mg– of In(OAc)3) with MPTS (5 mL) at room temperature. Then, the temperature was raised to 120 °C and the reaction mixture was kept under stirring for 15 min in order to allow the complete dissolution of the salts under N2. The solution

became

clear

and

slightly

yellow.

Afterwards,

the

reaction mixture was heated to the target temperature (180, 190, or 200 °C, as measured inside the reaction mixture) within 30 min.


8

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For all the syntheses, the timing was initiated as soon as the temperature inside the flask reached 180 °C, at which point the solution started becoming more intense in color. The solution color changed progressively from yellow to orange, then red, and finally brown. Aliquots were removed at predetermined time intervals of 30, 60, 90, 120, 180, 240, and 300 min. After 360 min, the flask was quenched in cold water. The samples were purified by dispersing the QDs in chloroform and subsequently precipitating them with ethanol. After collecting QDs by means of centrifugation, they were re-dispersed in chloroform or in an organic solvent such as acetone, dimethyl sulfoxide (DMSO), or tetrahydrofuran (THF). The samples were named according to their reaction temperatures as C180, C190, and C200. A synthesis at 210 °C was also attempted; however,

the

obtained

QDs

featured

poor

colloidal

stability,

likely due to the use of a temperature very close to the boiling point of MPTS, which might have favored the decomposition of the molecules.

Synthesis of CIS/ZnS QDs (core/shell) The epitaxial growth of a ZnS passivating shell was performed as a one-pot reaction. Parent CIS core QDs were synthesized according to the procedure described above for C200. After 360 min of reaction, a small aliquot was sampled, to compare the properties of core and core/shell architectures, and 0.8 mmol –146.8 mg– of


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Page 10 of 51

Zn(OAc)2 was swiftly added to the flask. Upon addition of the salt, the reaction mixture become lighter in color. Aliquots were sampled at 5, 10, 20, and 30 min. After 45 min, the heating was stopped and the flask quenched in cold water. The sample was purified by dispersing the QDs in chloroform and subsequently precipitating them

with

ethanol.

After

collecting

the

QDs

by

means

of

centrifugation, they were re-dispersed in chloroform or acetone.

Preparation of CIS QDs composites QDs-decorated silica nanoparticles. Silica nanoparticles (SNPs) were synthesized according to a modified Stöber method.27 Briefly, 14 mL of ethanol, 0.35 mL of NH4OH and 1.5 mL of water was mixed at room temperature in a 50 mL round-bottomed flask. After 10 min of stirring, 0.76 mL of TEOS was added dropwise. The mixture started becoming milky after few hours and was stirred overnight. The obtained SNPs were collected by means of centrifugation (10000 rpm for 30 min) and washed twice with a mixture of water and ethanol (1:1) and once with pure ethanol. The product was redispersed in 10 mL of ethanol for further characterization and modification. For the SNPs decoration, 1 mL of crude synthesized QDs’ sol was purified

according

to

the

procedure

outlined

above

and

the

particles were dispersed in 1 mL of chloroform. Concurrently, 1 mL of SNPs in ethanol was precipitated and re-dispersed in 3 mL of


10

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chloroform. QDs and SNPs were mixed in a 50 mL three neck roundbottomed flask and diluted to 10 mL with chloroform. The mixture was heated to 60 °C and stirred overnight. The SNP-QD composites were

centrifuged

(10000

rpm

for

30

min),

washed

twice

with

chloroform and once with ethanol before being re-dispersed in 5 mL of water. A 2.5 mL aliquot was transferred to a 3 mL glass vial and stirred with 8 mg of PEG-SH for 45 min. The particles were centrifuged (10000 rpm for 30 min) and re-dispersed in 2.5 mL of water.

Luminescent polymeric film. To prepare an inexpensive polymeric film readily applicable for contactless luminescence thermometry, 200 μL of crude CIS QDs’ sol was purified and re-dispersed in 200 μL of chloroform. The sol was mixed with 400 μL of store-bought transparent nail polish, vortexed for 30 s and 20 μL of the mixture was cast on a copper sample holder. The nail polish was allowed to set and dry overnight before performing luminescence thermometry measurements.

Characterization Structure, morphology, and composition The microstructure of the samples was investigated by means of X-Ray Powder Diffraction (XRPD) using a Philips diffractometer with a PW 1319 goniometer with Bragg-Brentano geometry, connected


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Page 12 of 51

to a highly-stabilized generator (40 kV). A focusing graphite monochromator

and

discriminator

were

a

proportional used,

counter

employing

a

with

a

pulse-height

nickel-filtered

Cu

K

radiation and a step-by-step technique (steps of 0.05° in 2 θ ), with a collection time of 10 sec/step. Fourier Transform Infrared (FTIR) spectra were recorded with a NEXUS-FT-IR

instrument

implementing

a

Nicolet

AVATAR

Diffuse

Reflectance accessory, using a resolution of 1 cm-1 and averaging the spectrum 56 times. The chemical composition of the samples was studied by means of inductively coupled plasma mass spectroscopy (ICP-MS)

measurements,

utilizing

a

PerkinElmer

Elan

6100

instrument. The samples were weighed and digested with a solution of 3 mL H2O, 3 mL aqua regia (HNO3:HCl = 1:3), and 1.5 mL HF (NOTE: HF is a dangerous chemical and care has to be exercised during its handling wearing proper gear and operating under a fume hood). The mixture was subjected to two microwave digestion cycles (CEM discover SPD). After the first cycle, 7.5 mL H2BO4 was added to neutralize HF. The solution was eventually filtered and diluted to 1:5 with a 2%aq HNO3. Cu+ and In3+ were quantified monitoring Cu (63 and 65 m/z) and In (115 m/z) channels. Scanning Transmission Electron Microscopy (STEM) was performed with a FEI Tecnai F20 instrument, equipped with a Schottky emitter and operated at 200 keV in High Angle Annular Dark Field (HAADF) mode. Energy Dispersive Spectrometry (EDS) analysis was performed


12

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by means of a Si-Li EDAX Phoenix spectrometer equipped with an ultra-thin window detector. JEOL JEM3010 Transmission Electron Microscope operating at 300 kV was used to acquire High Resolution Electron Microscopy (HRTEM) images and diffraction patterns from individual nanoparticles by means of the Nano Beam Diffraction (NBD) mode; a beam diameter of 5 nm was used with the smallest condenser aperture. The diffraction patterns were indexed like a Selected Area Diffraction (SAD) pattern.28 The surface chemical composition was investigated by means of Xray

Photoelectron

Spectroscopy

(XPS),

using

a

PHI

5600-ci

spectrometer (Physical Electronics, Eden Prairie, MN). The main XPS chamber was maintained at a base pressure below 8·10-9 Torr. A standard aluminum X-ray source (Al K = 1486.6 eV) was used to record the survey spectra (1400-0 eV, 10 min) and a standard magnesium

source

was

used

for

high-resolution

spectra,

both

without charge neutralization. The detection angle was set at 45° with respect to the normal of the surface and the analyzed area was 0.05 cm2. The

29Si

nuclear magnetic resonance (NMR) spectrum of the sample

C200 was recorded at room temperature on a Bruker Avance-500 spectrometer operating at 11.7 T (99.3MH for

29Si)

using a 4 mm CP-

MAS Bruker probe. The sample was packed in a 4 mm zirconia rotor and

measured

with

a

spinning

frequency

of

8000

Hz.

Direct


13


Excitation Magic Angle Spinning (DE-MAS)

29Si

Page 14 of 51

spectra were recorded

using the following acquisition parameters: 60 s relaxation delay, 3

µs

(90°)

excitation

pulse,

52

ms

acquisition

time.

Cross

Polarization Magic Angle Spinning (CP-MAS) spectra were recorded using a 5 s relaxation delay and 5 ms contact time. The processing comprised exponential multiplication of the free induction decay (FID) with a line broadening factor (lb) of 30 Hz, zero-filling, Fourier transform, phase and baseline corrections. The chemical shifts were calibrated with respect to tetramethylsilane (0 ppm). 13C

The

NMR spectrum of the sample C200 was recorded using the

abovementioned setup at a spinning frequency of 8000 Hz. CP-MAS spectra were recorded using a 5 s relaxation delay and 2 ms contact time. The processing comprised exponential multiplication of the FID with a lb of 30 Hz, zero-filling, Fourier transform, phase and baseline corrections. The chemical shifts were calibrated with respect to Adamantane (29.45 and 38.48 ppm). For comparison, the 13C

DE NMR and

29Si

DE NMR spectra of MPTS were recorded under

static conditions filling the rotor with the pure liquid precursor. Optical properties The aliquots sampled during the core and core/shell QDs growth were

dispersed

in

acetone

or

chloroform

without

further

purification prior to optical analyses. Photoluminescence

(PL)

measurements

were

performed

using

a

Horiba-Jobin Yvon Fluorolog 3-21 spectrofluorimeter. A Xenon arc


14

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lamp (450 W) was used as a continuous-spectrum source to record the PL spectra, selecting the excitation wavelength with a double Czerny-Turner monochromator. The spectra were recorded with 1 nm band pass resolution at room temperature, diluting the QDs to an optical density below 0.1 at the excitation wavelength (390 nm) in order to minimize self-absorption phenomena. Lifetime setups.

(LT)

The

measurements

different

were

core

CIS

conducted QDs

were

on

two

measured

different at

room

temperature under excitation at 373 nm using a NanoLed light source. The detection system consisted of an iHR300 single grating monochromator coupled to a R928 Hamamatsu photomultiplier tube (time resolution with the employed setup of approximately 200 ps). For the study of the core/shell system, emission intensity decay curves were obtained with an Edinburgh FLS920 spectrofluorimeter equipped with a Hamamatsu H 73-04 phototube and a TCC900 card for data

acquisition

in

time-correlated

single-photon

counting

experiments by using a PicoQuant LDH-P-C-405 pulsed diode laser as an excitation source (time resolution with the employed setup of approximately 200 ps). All the decay curves were fit with triexponential functions and the average lifetime was obtained as the weighted average of the three lifetime components according to the equation: 3

𝑟=

∑1𝑎𝑖𝜏𝑖 3

∑1𝑎𝑖

Eq. 1


15


The

absorption

spectra

were

Page 16 of 51

recorded

on

a

UV

Agilent

8453

spectrophotometer. The spectra were recorded in the 300-1000 nm range with a 1 nm resolution. The PLQY was measured using an aqueous solution of Ru(bpy)3Cl2 as a standard (emission peak: 613 nm, PLQY = 0.042 ± 0.002).29 Luminescence thermometry measurements of polymer film + QDs composite were performed using an in-house experimental set-up. The spectra were recorded with a Horiba Scientific Symphony Silicon CCD Detector 1024x256 (Horiba, New Jersey, USA) exciting the sample with a 405 nm 200 mW InGaN/GaN quantum well diode laser (Dragon Lasers, China) and focusing the excitation beam on the sample with a 10 cm-focus UV-fused silica concave lens. The temperature was lowered to 77 K and gradually increased to 340 K. For each step, upon

reaching

the

target

guarantee

thermal

collected

epifluorescently

temperature,

stabilization.

The

through

5 PL

the

min

were

emission same

waited signal

lens

and

to was the

excitation light was filtered off using a long-pass filter (> 535 nm – Thorlabs, USA). The spectra at each temperature were acquired in triplicate over two independent cycles.

RESULTS AND DISCUSSION Structural, morphological, and chemical characterization of core CIS QDs


16

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We report the characterization of the core CIS QD sample - C200 - reacted for 360 min, which is representative of the entire library

of

CIS

QDs

studied.

The

particles

had

a

tetragonal

chalcopyrite crystalline structure (I-42d - PDF #00-047-1372), as confirmed by their diffractogram (Figure 1A) and the NBD analysis (Figure 1B). STEM observations provided additional evidence

Figure 1. Characterization of the core CIS QD sample C200, obtained after 360 min of reaction. XRPD pattern of the QDs along with the reference pattern for the chalcopyrite structure (A). NBD performed on an area where few NPs where present, the presence of spots confirms the crystalline nature of the QDs and their distances correspond to the {024} (red *) and {112} (white *) family planes of chalcopyrite (B). STEM image (C) along with size distribution (mean size of 2.2 nm – bottom inset in C) and representation of the tetrahedral habitus of the QDs (upper inset in C) – characteristic of the chalcopyrite polymorph. EDS spectrum (D) obtained from the area in C showing the expected


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Page 18 of 51

signals from the elements present in the QD core (Cu, In, S) along with the strong signals from MPTS (Si, C, O, and S). The signal from nickel (Ni), coming from the grid, overlaps with that of copper. XPS survey spectrum of C200 (E). ssNMR spectra of C200 (red line) along with that of pure MPTS (black line) (F).

for the assignment of the crystalline phase, since the QDs have a tetrahedral

morphology

(Figure

1C),

which

is

typical

of

QDs

crystallized in the chalcopyrite polymorph.30-31 The mean size of the QDs, obtained considering the edge of the observed tetrahedra, was 2.2 ± 0.3 nm (inset in Figure 1C). The presence of a thick layer of MPTS molecules around the QDs contributed as a background to the HRTEM images significantly reducing the signal-to-noise ratio, thus preventing the direct observation of the lattice planes of QDs. For this reason, the crystallinity of single QDs have been verified by means of NBD that allows to obtain diffraction patterns from single nanoparticles. EDS measurements (Figure 1D), as well as XPS analysis (Figure 1E and S1), returned all the expected elements (Cu, In, S, and Si). The atomic composition obtained from the XPS spectra – rich in C, O, and S – again suggested a considerable presence of MPTS molecules on the surface of QDs compared to the metals (In and Cu). These MPTS molecules remained tethered to the QD surface after the reaction, imparting colloidal stability.

In

the

FTIR

spectrum

of

C200

(Figure

S2),

the

characteristic vibrations of MPTS molecules could be observed thus


18

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confirming the presence of these molecules on the QD surface. The broad vibration above 3200 cm-1 arose from OH stretching and was related to the presence of silanol groups on the QD surface, due to the hydrolysis of silane molecules. These OH groups could condense during the thermal treatment with consequent formation of Si-O-Si bridges, as confirmed from

29Si

ssNMR (Figure 1F). This

analysis revealed the presence of a combination of signals centered at -67.3, -58.2 and -48.8 ppm, which were assigned to -CH2-Si(OSi)3 (T3), -CH2-Si(OSi)2(OH) (T2) and -CH2-Si(OSi)(OH)2 (T1) species, respectively. The absence of T0 contributions (usually found around -40 ppm), which would correspond to uncondensed MPTS precursor, clearly indicated the occurrence of condensation phenomena at the 13C

surface of the QDs. signals

ascribed

to

ssNMR measurements showed patterns of MPTS-related

compounds,

along

with

contributions likely from other species formed in situ via thermal decomposition

of

the

precursors

(Figure

S3).

Finally,

ICP-MS

measurements revealed that C200 was heavily copper-deficient with a Cu-to-In ratio of 0.49 (similarly C180 and C190 featured a Cu/In ratio of 0.52 and 0.47, respectively). These results suggested a stronger

reactivity

of

In3+

in

the

reaction

environment,

in

accordance with the poor affinity of thiol ligands (soft Lewis


19


Page 20 of 51

bases) with In3+ ions (hard Lewis acid), ultimately leading to

Figure 2. Steady-state optical characterization of CIS QDs. Absorption spectra (A and C), corresponding Tauc plots (insets in A and C), and emission spectra (B and D) of QDs obtained varying the reaction time (Treaction = 200 °C – C200) and temperature (treaction = 360 min). In the inset of B, a colloid of C200 in acetone under UV excitation (approx. 365 nm) shows intense red luminescence. Dashed grey lines in the insets in A and C are linear fits to the Tauc plots.

an imbalanced availability of the two cations. However, it is possible that some unreacted reagents – that were not completely washed

out

–

stoichiometry.

contributed

to

The

deficiency

copper

the

determination of

these

of

the

QD

samples

is


20

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desirable, since such composition is known to lead to a brighter QDs compared to their stoichiometric counterparts.9,

32

Optical characterization The absorption spectra of C200 core CIS QDs showed multiple weak features (Figure 2A and 2C) and a band gap far below the tabulated value

(815

nm)

of

the

bulk

material.33 This

latter

evidence

indicated the existence of a strong quantum confinement regime, as expected from the small size of QDs. The emission profiles were asymmetrical and rather broad, with a full width at half maximum (FWHM) of approximately 120-130 nm (Figure 2B and 2D). The PL peaks were centered between 650-680 nm, with a large Stokes shift (up to approximately

90

nm).

characterize CIS QDs.21,

Altogether, 34

these

features

usually

Both an increase of the reaction time

and temperature led to a shift of the absorption onset towards longer wavelengths, due to the growth of the QDs yielding a band gap narrowing. The use of the Tauc approach35 allowed following quantitatively the band gap variation (insets in Figure 2A and 2C). The obtained band gap values were used to indirectly determine the

QD

size

according

to

the

effective

mass

approximation,6

obtaining values well in accordance with TEM observations (Table S2). LT measurements showed a PL decay rate in the hundreds of nanoseconds range (Figure 3 and Table 1). In particular, a higher reaction temperature corresponded to a slower PL decay (Figure


21


Page 22 of 51

3A). Usually, LT and PLQY are directly correlated. Indeed, we observed that a longer reaction time corresponded to a higher emission efficiency of the QDs (see Table 1).36 Such correlation stems from the fact that poorly crystallized and smaller QDs feature higher defect density and larger surface-to-volume ratio, resulting in a large number of trap states for the charge carriers. This characteristic leads to a higher probability of non-radiative decay events, which are responsible for the mean luminescence LT decrease and the PL emission quenching. Size polydispersion and inhomogeneity

of

the

recombination

centers31

play

a

role

in

determining the non-monoexponential photoluminescence decay, with surface faster

trap-related components

of

emission the

mechanisms

kinetics.37-38

contributing

Due

to

these

to

the

reasons,

assignation of specific lifetime

Figure 3. PL decay curves for QDs obtained after 360 min of reaction at different temperatures (monitoring the emission at the PL peak maximum – A), and specifically for the sample C200 monitored throughout the emission profile


22

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(B). Inset in A shows a zoom-in of the decay curves at short times. Inset in B shows the PL emission profile of the investigated sample.

Table 1. Results of the decay curve fits and PLQY of the QDs. The decays were monitored at the maximum of the emission profile for the batches synthesized at different temperatures. The monitored wavelength scan was performed on the sample C200. Each average lifetime was obtained according to Equation 1. The PLQY was calculated comparing the QD emission with that of the standard Ru(bpy)3Cl2 (emission peak: 613 nm, PLQY = 0.042±0.002). The relative error associated to the average lifetime values is approximately 1%, as obtained from the fitting procedure of the decay curves. The coefficient of determination (R2) was at least 0.999 for all fitting curves.

Sample

, ns

PLQY, %

83

0.34

Name C180

±

0.02 C190

124

1.50

±

0.08 C200

143

6.7

±

0.3 Wavelength , ns 600 nm

97

660 nm

143

740 nm

213

components to each de-excitation mechanism is not possible. Only the average lifetime value is a meaningful parameter to describe the behavior of the particle ensemble and to compare batch-tobatch

differences.

The

PL

decay

of

C200

was

also

monitored


23


Page 24 of 51

throughout the emission profile, observing that the longer the emission

wavelength

Similarly,

the

the

slower

excitation

the

spectra

decay

recorded

rate

(Figure

monitoring

3B).

the

PL

signal of sample C200 throughout its emission profile (Figure S4) showed that the longer the emission wavelength the more red-shifted the exciton peak. This behavior is typical of QDs with a non-sharp size distribution,39 but also heterogeneity in the distribution of defects (i.e.,

emitting centers in

CuInS2 QDs) could

heavily

contribute to this effect.31 Solvent effect The

MPTS-passivated

QDs

were

dispersible

in

solvents

with

different polarity indexes. This characteristic is very appealing when considering their possible application in the framework of device production. After the purification, the QDs were dispersed in chloroform to obtain an optically clear sol. This sol was used to obtain an aqueous dispersion of CTAB-stabilized QDs using a well-established approach.40 Further experiments are ongoing to replace

CTAB

biological possibility

with

another

applications; to

exploit

moiety

also, such

we

that are

cationic

is

more

currently surfactant

amenable

to

testing

the

to

grow

a

mesoporous silica layer around the QDs. The crude reaction product was also easily dispersed in acetone, dimethyl sulfoxide (DMSO) and tetrahydrofuran (THF) (Figure 4A). After purification, the colloidal stability of QDs in these solvents is improved upon


24

Page 25 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


addition

of

mercaptoethanol

(Figure

S5).

The

role

of

mercaptoethanol is considered to be the formation of disulfide bridges with thiol moieties belonging to MPTS

Figure 4. Core CIS QD sample C200 can be dispersed in solvents with different polarities retaining its optical features (A – 20 μL of crude reaction product in 2 mL of different solvents). The absorption and emission profiles of the various colloids under 440 nm excitation are shown in B. The emission peak maximum red-shifts depending on the polarity of the solvent, following the Reichardt’s empirical parameter (C). Color code in B and C is the same.

molecules. In this way, the outer surface of the QDs features an increased

amount

of

hydroxyl

groups

(see

discussion

in

the

Supporting Information). The colloids obtained in the mentioned solvents displayed variable optical emission efficiencies (Figure 4B); water and DMSO more markedly quenched the emission of QDs. Nonetheless, the emission in all the colloids was still detectable. We

also

observed

a

solvatochromic

effect

in

the

different

dispersions, since an increase of the solvent polarity leads to a


25


Page 26 of 51

bathochromic shift of the emission of photoluminescent species in dispersion.41 This follows from more efficient solvent-related nonradiative relaxation processes. A parameter that is commonly used to describe the polarity of a solvent is the so-called Reichardt’s polarity

parameter

solvatochromic

ET(30),

properties

of

which the

is dye

estimated

from

the

2,6-diphenyl-4-(2,4,6-

triphenylpyridinium-1-yl)phenolate (betaine 30).42 By plotting the wavelength of the QD PL maximum in the different solvents versus the polarity parameter, we observed a direct dependence between the two quantities (Figure 4C). The good passivation


26

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Figure 5. Absorption (A) and PL emission (B) spectra of CIS QDs at different stages of ZnS shell growth onto C200 cores. In C, the variation of the PLQY is shown. Decay curves of the parent core (C200) and core/shell QDs after 45 min of shell growth (D). The PL emission spectra and the decay curves were recorded in chloroform under 405 nm excitation. The decay curves were recorded monitoring the signal at the emission profile maximum.

imparted by the MPTS molecules on the surface of the QDs is confirmed by the fact that the PL signal of a QD sol in acetone (in the absence of mercaptoethanol) retained 90% of the initial intensity upon continuous irradiation for 180 min with UV light (Figure S6). Effect of the shell growth The growth of a ZnS shell on core CIS QDs had a huge impact on the optical properties of the QDs (Figure 5). The creation of a CIS/ZnS

architecture

proceeds

according

to

a

partial

cation

exchange process that leads to the presence of an intermediate CuZn-In-S alloyed layer between the

actual

CIS

core

and

the

ZnS

shell.43

This

mechanism

is

responsible for a partial shrinkage of the actual core size that is believed to be the reason for the overall blue-shift of the excitonic

absorption

and

PL

emission

(Figure

5A

and

5B,

respectively).44-45 The formation of the above mentioned alloyed layer might also contribute to this shift (Eg,CIS = 1.5 eV and Eg,ZnS = 3.54 eV). In our sample, this shift was noticeable at the very


27


Page 28 of 51

beginning of the shell growth stage and becomes less appreciable upon

extending

the

reaction

time.

The

observed

behavior

is

consistent with a process involving a first step of cation exchange followed by the formation of a thicker ZnS shell that hinders further core etching/alloying. Remarkably, after 45 min of shell growth, the PLQY increases from approximately 6% to about 55% (Figure 5C) with an overall blue-shift of the emission peak maximum of 25 nm. This ten-fold increase in the PLQY stemmed from the shielding of the parent core QDs from the outer environment and the surface passivation, which removed a great part of surface defects responsible for electron trapping.46-47 The fact that the emission of the prepared QDs was influenced by the presence of surface

states

was

confirmed

by

the

decay

curves

recorded

monitoring the PL signal of core and final core/shell CIS QDs (Figure 5D). Notably, the core/shell architecture featured a much slower decay rate compared to core CIS QDs, with a mean lifetime of 273 and 136 ns, respectively. This is due to the partial suppression of the short-lived contribution coming from the nonradiative recombination pathway for which the surface states are responsible. Overall, the ZnS shell growth allows to improve the optical performances system

of

appealing

MPTS-passivated also

for

CIS

QDs,

applications

making

were

the

proposed

brightness

is

a

stringent requirement.


28

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Nanocomposite systems The C200 core QDs were used to prepare two nanocomposite materials, showcasing the versatility of the developed material. On one side, the presence of thiolated silica precursor molecules on the surface of QDs makes it straightforward to decorate silica-based systems with such photoluminescent moieties. In this specific instance, silica nanoparticles (SNPs) were prepared and simply mixed with QDs

in

chloroform.

Upon

heating

the

mixed

dispersion,

MPTS

molecules underwent condensation on the surface of SNPs yielding QD-decorated SNPs (Figure 6A). The obtained nanocomposite did not display satisfactory colloidal stability in water, with noticeable aggregation over the course of few minutes (Figure S7). This behavior was directly related to the presence of unsaturated thiol groups that imparted hydrophobicity to the system, as discussed in previous

sections.

thiolated

This

polyethylene

impasse

was

overcome

glycol

molecules

in

by

introducing

the

unstable

suspension. These species tethered to the surface of the QDs via disulfide bonds, imparting colloidal stability to the whole system (Figure 6B). The optical properties were retained by the QDs upon blue

light

excitation

(405

nm)

with

a

minimal

shift

of

the

nanocomposite emission compared to the PL of the parent CIS QD sol in chloroform (inset in Figure 6B). The stunning simplicity of


29


Page 30 of 51

this approach makes it applicable in the preparation of a variety of luminescent silica-based systems. In the context of biomedical applications, the biocompatibility of nanoparticles – such as magnetic beads,48-49 plasmonic materials50-51 and lanthanide-doped particles52 to name a few – is often increased upon coating them with

silica.

Silanization

is also

a means to

facilitate the

tethering of additional moieties to the particles’ surface to create multifunctional systems.53-54 It is evident that, following the presented method of decoration with QDs, PL properties can be imparted to a virtually infinite variety of systems, thus obtaining desired multifunctional probes.


30

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Figure 6. Characterization of the nanocomposites. Silica nanoparticles (SNPs) obtained via a modified Stöber approach before and after decoration with MPTSpassivated CIS QDs (A – 50 nm scale bars). (B), suspensions of SNPs and thiolated polyethylene glycol-stabilized SNPs decorated with QDs under daylight (left) and 405 nm excitation (right – image taken with a 500 nm long pass filter). In the inset in B, the normalized spectra of a QD sol in chloroform and the SNPs decorated with QDs suspended in water are presented. (C), Temperature-dependent emission of a polymeric film embedding MPTS-passivated CIS QDs under 405 nm excitation (solid lines) along with exemplary Gaussian curves used for the deconvolution of the signal recorded at 250 K. In the inset in C, the polymeric film under daylight and UV light is shown. Relative thermal sensitivity of the thermometric approach (D) and corresponding thermal parameter (Δ - inset in D) over the investigated temperature range.

Additionally, the compatibility of these MPTS-passivated CIS QDs with polar media gives the opportunity to embed them in polymer matrices. Indeed, we mixed a chloroform sol of the presented CIS QDs with store-bought transparent nail polish. Notoriously, nail polish is composed of nitrocellulose – a highly polar polymer – as the film maker, usually mixed with organic polar solvents such as various acetates (mainly ethyl and butyl acetate) and toluene. The polarity of nitrocellulose, imparted by nitro groups, does not impede the dispersion of MPTS-passivated QDs in this polymeric matrix while retaining their optical properties (Figure S8). This observation opens new possibilities in terms of polymeric matrices


31


Page 32 of 51

that can be used for the production of composite devices of practical interest. In

the

frame

luminescent

of

functional

nanoparticles,

systems

the

obtainable

preparation

of

exploiting contactless

luminescence nanothermometers is a field that holds great promise given the achievable sub-micrometric spatial resolution of the temperature

readout.

Not

only

luminescence

thermometry

is

of

interest for biomedical in vivo and in vitro applications55-56 but also for gaining insight in fundamental properties of the matter in the nanoscale realm57-58 and for real-time thermal monitoring of miniaturized electronic devices.59-60 In the two latter contexts, the use of readily available and inexpensive UV/blue excitation light sources does not represent a functional drawback. This holds true also for in vitro bio-applications, as opposed to in vivo biomedical

applications

where

high

penetration

depth

is

a

requirement effectively matched by the use of fully near-infrared light

operating

systems.55,

61

Therefore,

the

inexpensive

luminescent polymeric film, here obtained with CIS QDs, represents an appealing solution to readily perform thermal monitoring. Although temperature-dependent variations of the emission of CIS QDs

have

already

been

observed,62

their

exploitation

in

luminescence thermometry is yet to be reported. Specifically, we performed luminescence thermometry over a broad temperature range exploiting the broadening and red-shift of the nanocomposite’s


32

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emission profile occurring when passing from 140 to 340 K (Figure 6C). Both spectral variations are suitable to obtain a temperature readout

from

a

luminescent

species.63-64

In

sharply

emitting

lanthanide-based systems, the profile broadening is a convenient parameter to consider for luminescence thermometry.65 However, in QDs the absolute variation of the profile width is small compared to the inherently broad emission. This makes a thermometer based on the profile width poorly sensitive for the proposed system (Figure S9). A better performance in terms of thermal sensitivity was obtained considering the emission spectral shift (Figure 6D). As pointed out by Carlos and co-workers,66 a deconvolution step in the case of signals composed of overlapping components leads to a more

accurate description

of the

profile

and

hence

a higher

sensitivity. In the case under study, the emission profile was deconvoluted using three Gaussian curves centered at 642, 676 and 735 nm. The position of these components was chosen upon fitting the emission profile of the polymeric film at the intermediate temperature of 250 K, and it was kept constant for the rest of the fitting procedures. The thermal parameter (Δ) was obtained as the ratio between the sum of integrated area of the two red-most components (2) + (3), and the one centered at 642 nm (1). This approach was found to be the most convenient among the ones tested to account for the peak shift and in terms of final sensitivity obtained. An accurate mechanistic description of the emission


33


Page 34 of 51

based on the attribution of the three components to different emission processes lies beyond the scope of the study. In fact, the deconvolution was merely utilized to reliably account for the red-shift experienced by the embedded QDs’ emission. The variation of Δ depending on the temperature was fitted using a built-in Origin® Boltzmann function and the corresponding relative thermal sensitivity was obtained from the numerical derivative of this function according to the equation:64 𝑆𝑟 =

1∂∆ ∆∂𝑇

Eq. 2

The sensitivity displayed by the system is above 1% K-1 for temperatures higher than 280 K, hence making these particles of particular interest for in vitro luminescence nanothermometry in the temperature range of biological interest. Remarkably, the sensitivity displayed by this system is on par67 or higher68-70 when compared

to

luminescence

the

values

featured

thermometers,

with

by the

state-of-the-art additional

QD-based

advantage

of

avoiding the presence of toxic metals that characterize previously reported systems (lead and cadmium above all). All this considered, the proposed MPTS-passivated QDs represent a system with a great potential for application in a spectrum of fields comprising (but not limited to) fluorescent bio-labels, luminescent

polymeric

films

and

contactless

luminescence

thermometry.


34

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CONCLUSIONS We

reported

decomposition

the

synthesis

approach

of

involving

CuInS2 the

QDs use

using of

a

thermal

mercaptopropyl

trimethoxysilane (MPTS), which simultaneously plays the role of chelating agent and sulfur source. The obtained QDs have a maximum PLQY of approximately 6%, which can be improved by almost an order of magnitude (55%) upon growing a ZnS shell on the parent CIS QDs core. The PL signal of the proposed MPTS-passivated CIS QDs shows good stability under continuous UV illumination. Notably, the QDs are dispersible in chloroform and in a number of hydrogen-bonding solvents such as acetone, tetrahydrofuran, and dimethyl sulfoxide retaining their characteristic emission. All these features stem from a complex surface chemistry exhibiting MPTS molecules that underwent condensation, thus resulting in a silanated passivating layer featuring siloxane groups. Altogether, said characteristics make these QDs highly flexible, opening new avenues in terms of possible applications that require dispersibility in different media and state-of-the-art optical performance.

We

further

demonstrated

the

versatility

of

this

system by preparing two different nanocomposites, namely CIS QDdecorated silica nanoparticles and a luminescent polymeric film embedding CIS QDs.

The

latter

composite

was

used

to perform

luminescence thermometry over a broad temperature range (140-


35


Page 36 of 51

340 K), harnessing the temperature-dependent luminescence of these heavy-metal free QDs.

ASSOCIATED CONTENT Supporting

Information.

characterization

(XPS,

FTIR,

additional ssNMR)

of

C200

physicochemical CIS

QDs;

size

estimation using the effective mass approximation; wavelengthdependent excitation spectra of C200; study of the colloidal and emission stability of C200; additional characterization of the nanocomposites (pictures of the SNPs-QDs composite suspension, absorption spectrum of the luminescent polymeric film, SEM of the polymeric film, thermometric approach based on peak broadening). Corresponding Author * Patrizia Canton, [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors thank Dr. Antonio Benayas for the fruitful discussions, Mr. Tiziano Finotto for the XRD measurements, and Mr. Nicolò Mazzucco for the ICP-MS analyses. ADIR 2015 Grant from Ca’ Foscari


36

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University of Venice is gratefully acknowledged. Prof. Fiorenzo Vetrone

is

grateful

to

the

Natural

Sciences

and

Engineering

Research Council (NSERC) of Canada and the Fonds de recherché du Québec

-

Nature

et

technologies

(FRQNT)

for

supporting

his

research. Artiom Skripka is grateful to FRQNT for financial support in the form of a scholarship for doctoral studies (Bourses de doctorat en rechérche). Dr. Luca Fusaro is acknowledged for his support in the ssNMR measurements; this research used resources of the nuclear magnetic resonance service located at the University of Namur. This service is a member of the “Plateforme Technologique Physico-Chemical Characterization” – PC2

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Table of Content Artwork


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Scheme 1. Synthesis of the MPTS-passivated CuInS2 QDs (top) and their use in the preparation of two nanocomposites (bottom left – luminescent polymeric film for luminescence thermometry; bottom right – silica nanoparticles decorated with QDs). 216x247mm (150 x 150 DPI)



Figure 1. Characterization of the core CIS QD sample C200, obtained after 360 min of reaction. XRPD pattern of the QDs along with the reference pattern for the chalcopyrite structure (A). NBD performed on an area where few NPs where present, the presence of spots confirms the crystalline nature of the QDs and their distances correspond to the {024} (red *) and {112} (white *) family planes of chalcopyrite (B). STEM image (C) along with size distribution (mean size of 2.2 nm – bottom inset in C) and representation of the tetrahedral habitus of the QDs (upper inset in C) – characteristic of the chalcopyrite polymorph. EDS spectrum (D) obtained from the area in C showing the expected signals from the elements present in the QD core (Cu, In, S) along with the strong signals from MPTS (Si, C, O, and S). The signal from nickel (Ni), coming from the grid, overlaps with that of copper. XPS survey spectrum of C200 (E). ssNMR spectra of C200 (red line) along with that of pure MPTS (black line) (F). 383x230mm (150 x 150 DPI)


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Figure 2. Steady-state optical characterization of CIS QDs. Absorption spectra (A and C), corresponding Tauc plots (insets in A and C), and emission spectra (B and D) of QDs obtained varying the reaction time (Treaction = 200 °C – C200) and temperature (treaction = 360 min). In the inset of B, a colloid of C200 in acetone under UV excitation (approx. 365 nm) shows intense red luminescence. Dashed grey lines in the insets in A and C are linear fits to the Tauc plots. 274x196mm (150 x 150 DPI)



Figure 3. PL decay curves for QDs obtained after 360 min of reaction at different temperatures (monitoring the emission at the PL peak maximum – A), and specifically for the sample C200 monitored throughout the emission profile (B). Inset in A shows a zoom-in of the decay curves at short times. Inset in B shows the PL emission profile of the investigated sample. 302x129mm (150 x 150 DPI)


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Figure 4. Core CIS QD sample C200 can be dispersed in solvents with different polarities retaining its optical features (A – 20 μL of crude reaction product in 2 mL of different solvents). The absorption and emission profiles of the various colloids under 440 nm excitation are shown in B. The emission peak maximum redshifts depending on the polarity of the solvent, following the Reichardt’s empirical parameter (C). Color code in B and C is the same. 271x104mm (150 x 150 DPI)



Figure 5. Absorption (A) and PL emission (B) spectra of CIS QDs at different stages of ZnS shell growth onto C200 cores. In C, the variation of the PLQY is shown. Decay curves of the parent core (C200) and core/shell QDs after 45 min of shell growth (D). The PL emission spectra and the decay curves were recorded in chloroform under 405 nm excitation. The decay curves were recorded monitoring the signal at the emission profile maximum. 227x198mm (150 x 150 DPI)


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Figure 6. Characterization of the nanocomposites. Silica nanoparticles (SNPs) obtained via a modified Stöber approach before and after decoration with MPTS-passivated CIS QDs (A – 50 nm scale bars). (B), suspensions of SNPs and thiolated polyethylene glycol-stabilized SNPs decorated with QDs under daylight (left) and 405 nm excitation (right – image taken with a 500 nm long pass filter). In the inset in B, the normalized spectra of a QD sol in chloroform and the SNPs decorated with QDs suspended in water are presented. (C), Temperature-dependent emission of a polymeric film embedding MPTS-passivated CIS QDs under 405 nm excitation (solid lines) along with exemplary Gaussian curves used for the deconvolution of the signal recorded at 250 K. In the inset in C, the polymeric film under daylight and UV light is shown. Relative thermal sensitivity of the thermometric approach (D) and corresponding thermal parameter (Δ inset in D) over the investigated temperature range. 317x218mm (150 x 150 DPI)


Mercaptosilane-Passivated CuInS2 Quantum Dots for Luminescence

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